145511306 belt tensioning

5/26/2018 145511306 Belt Tensioning

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Lets consider whats involved

In achieving the correct tension of a V-Be

Typical Construction of a Standard Duty V-Belt

1. Wear and Oil Resistant External Fabric

2. Compression Section

3. High Strength Tension Members


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Here are a few examples

of different belt designs

that can be applied to

varying applications:


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Take a closer look at what goes on.Pulley wear is very important

If the pulley is worn youre fighting a loosing batt

The belts V shape produces

more contact surface than a flat

belt can for a given pulley width.

The wedge shape helps the pulley

get a grip on the belt.

The bottom of the V swells

under the compression of the belt

being bent around the radius of the

pulley.

If the grooves of the pulley are worn

the contact area is largely reduced.

When a new belt is run on a worn

pulley, the areas of contact between

the pulley and the belt are at very

high pressures. The belt material

cannot withstand the excessive

compression so the belt material wi

quickly be worn away.

As the accelerated wear occurs, you

belt drive will begin slipping in sho

order. You will be called back to

increase the belt tension again to sto

the slipping.


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The Deflection Method is the best way to

QUANTIFYbelt tension.

A popular Rule of Thumb is to deflect the belt 1/64 per inch of center distance. That is to say, if thecenter distance between two shafts is 100 inches, the belts should be deflected 100/64 or 1 9/16. A

decimal equivalent chart is a big help in converting the measurements. You will always have a chart if yokeep your copy of the Electrical Engineering Pocket Handbooknearby.

The question then becomes, How much pressure should it take to deflect the belt thatmuch? This is where the fly gets into the ointment. The following table can be used as a starting point.

Recommended Tension for V-Belt Drives

Small Sheave Deflection Force in Lbs.

V-BeltSection

Speed Range Diameter Speed Ratio

1.0Speed Ratio

1.5Speed Ratio

2.0Speed Ratio

4.0 +

1800-3600 3.0 2.0 2.3 2.4 2.6

1800-3600 4.0 2.6 2.8 3.0 3.3

1800-3600 5.0 3.0 3.3 3.4 3.7A

1800-3600 7.0 3.5 3.7 3.8 4.3

1200-1800 4.6 3.7 4.3 4.5 5.0

1200-1800 5.0 4.1 4.6 4.8 5.6

1200-1800 6.0 4.8 5.3 5.5 6.3B

1200-1800 8.0 5.7 6.2 6.4 7.2

900-1800 7.0 6.5 7.0 8.0 9.0900-1800 9.0 8.0 9.0 10.0 11.0

900-1800 12.0 10.0 11.0 12.0 13.0C

700-1500 16.0 12.0 13.0 13.0 14.0

900-1500 12.0 13.0 15.0 16.0 17.0

900-1500 15.0 16.0 18.0 19.0 21.0

700-1200 18.0 19.0 21.0 22.0 24.0D

700-1200 22.0 22.0 23.0 24.0 26.0


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Of course you can remember all of those numbers.Right?

Well, dont worry, cause nobody else does either

In practice,I think you will find if you apply the criteria from the table above, your belwill be fiddle stringtight. When belts are run that tight, there is no question that theywill perform to their rated capacity. The belt people want you to be satisfied that their belt

will pull the load, and they want you to get them so tight that they cant slip.

The problem is though,in higherhorsepower, multiple belt applications,these deflection specifications can add up

to disaster. Remember, there are othermechanical concerns that the beltmanufacturer cant begin to take into

account. Concerns like will the shaftsbreak? Can the bearings stand up under

that much load? Will the heat generated in

such heavily loaded bearings cause the

grease to melt and run out. The bearingssurely wont last if that happens.

This motor shaft was broken due to

excessive loading.

Machines canbe built to withstand excessivebelt loads, but in most cases they are not. So

the onusis on you to do it right. Dont take the belt manufacturers tensioning tables ause them as Gospel, because they are only addressing belt performance. They are NOTaccounting for the limitations of your machinery.

Use the Deflection Method to achieve uniformityin your V-belt applications.will take good organization to establish what the correct amount of deflection is. Every

application needs to be treated separately, because every application is different. You willneed to establish records of successful belt drive installations and use those records inensuing repair and maintenance operations.


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In the final analysis, the correct belt tension is just enoughtension to keep the belt from slipping under normalload

conditions.

Please Notice the word normal now, because well want to talk more about thlater.

The determination of the correct tension is a matter of knowing the dynamics of theapplication, making some careful observations, and drawing the right conclusions.

The first stepin a successful v-belt installation is having some idea of the capacity of tbelt and pulley combination. If you know what the horsepower-per-beltrating is, you will

able to get a feeling for how much you are demanding from the belts. This information isavailable from the application data established by the belt manufacturer. In most cases, V-

belt application engineering has been done by the equipment manufacturer or your belt andpulley vendor. Its a good thing to ask who specified the components, because anybody ca

make a mistake, even an equipment manufacturer. You need to know who was involved,

because you need to have confidence in the finished installation.

There are different approaches to V-belt application engineering, and too often, COST

becomes a deciding factor in how conservatively, or aggressively V-belts and pulleys are

utilized.

Be advised that it costs more money to keep the applied horsepower per belt low, soif your

horsepower-per-belt ratio is at the higher end of the manufacturers established limits, youwill know that the belts will have to be awfully tightto transmit the load back to the motor.

If the belt drive is at the high end of the horsepower per belt ratio, or works out to only hava 1.0 Service Factor, I recommend that you ask whoever specified the components to go ba

to the drawing board, and get you some heavier duty stuff before you waste your time puttiit all together.


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The next step is to take control, and take responsibility for the tensioningoperation. Make a careful installation so you will be able to eliminate misalignment of the

belt drive. This is very important to the belts, and it is important to the bearings in yourmachine and to the motor. Misalignment can damage the cords in the belts, cause excessiv

wear and excess heat, as well as waste energy.

Multiple belt installations require careful alignment so that the belts can be tightened evenl

Accurate alignment is the part of the job

your supervisor doesnt want to deal with

either. So make him proud, dont askquestions, just go ahead and do it right.

Now is the time, to take the time, to give

the belts and your machine an even chancefor success.

Misalignment will significantly reduce the

capacity of the drive and demand higherbelt tensions in order to pull the load.

Use a Straight edge or a piece of string that you can draw across the face of the pulleys to

gauge the misalignment.


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Taper Lock Bushings will cause you a headache too, because the engagement ofthe taper and the offset of the edges of the bushing and pulley will vary from one pulley to

the next.

After you have resolved the initial angular misalignments,you will probablyhave to move one of the pulleys to correct any remaining parallel misalignment. Do yourse

a favor by taking some measurements while the pulleys are mounted.

Your straight edge will indicate how much thepulley has to move. This measurement is

indicated by the REDlines in the drawing to th

left.

In this particular instance, I would decide tomove the top pulley back further onto the shaf

This will get the pulleys closer to the machineand help to reduce bearing loads.

The BLUElines are the measurement from the

bushing to the end of the shaft. I would add thearlier pulley offset to the bushing measureme

and then position the top bushing accordingly,further onto the shaft. After the bushing is

retightened, the parallel alignment of the pulleshould be correct.

amount.


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When mounting the belts,dont force them onto the pulleys. Move the shafts cloenough together so there is plenty of slack to allow the belts to slide easily over the pulleys

Never STRETCH the belts when putting them on. If you get too rough putting the belts onthe pulleys, you could damage or even break the internal tension membersin the belts.

Damage to the tension members might cause the belts to flip over or even break when

they go into operation and encounter heavy loads.

Go ahead and tighten the beltsto something approaching what the manufacturer callfor in their table. Be sure to roll the pulleysas you get closer to the final tension setting. L

the belts feeltheir wayinto the pulley grooves as you increase the tension. Verify theaccuracy of your alignment as you go. Continue to take up the slack while maintaining eve

tension on all of the belts.

If the belts arent equally tensioned, find out why and make the necessaryadjustments.

Now youre ready to start the machine. Follow all of your normal safety precautioand then alert those working within the vicinity of the machine about your intentions to

attempt a start-up. Remove the lockouts and start the machine.

Listen and observe carefully.If you are NOT using some soft

start method for starting the

motor, dont be alarmed if the

belts Squawk during the

acceleration of a high inertia or

fan load. Normal, Design B,squirrel Cage motors develop 200%(or more) of their rated torque

during acceleration. Keep in mindthat your belt drive needs to be

rated for the motor and the load. Itdoesnt need to be rated for twice

your motors horsepower. In thecase of across the line starting,

its OK, and you can expect, thebelts to slip a little at some point

while the load is accelerating.

Dont think you need to totally eliminate belt

squawk when a motor starts.


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In the case of an existing installation,it is natural, due to belt stretch, to have to rtension a belt drive, but consider the possibility that something may have changed to increa

the load. Electric motors are capable of developing more power than their continuous ratinimplies, so if your belts are slipping, you should measure the motor currents before you

decide on what action to take. Re-tensioning the belt drive might not be the right thing to d

If the belts squeal or slip when the machine is at full speed and running at a full normal loashut it down and re-tighten the belts. Use the Deflection Method, and employ the data th

was learned during the initial start-up on the machine.

If you are working on a new installation,be sure and consider whether the machinwas running normally, or if it is encountering some unusual transient load. Be aware thanew start-ups may present overload situations that can be accounted for and eliminated onc

the process variables have all been examined and properly adjusted. Make sure that themotor is not being overloaded before you tighten the belts.

In any event, monitor bearing temperaturesbefore and aftermaking adjustments to beltdrives. Elevated bearing temperatures may be your best indicator of potential problems.

Ball bearings will run hot if they are under-greased, over-greased, or if they are overloadedMost ball bearings get warm when running, and a good rule of thumb is that normal

conditions should not generate bearing temperatures higher than about 150 degrees F at thehousing. If you see bearing housing temperatures over 170 degrees F, something is probab

wrong, or you are dealing with a very unusual application.

Know what the normal operating temperatures of the bearings are before making adjustmen

to the belt drive. It is a good idea to ask if someone has just added grease to the bearingsbefore you begin your work. If recent (excessive) relubrication is a possibility, check your

maintenance records to determine that the bearings are operating at their normal temperaturbefore re-tensioning the belt drive. If the bearing temperatures are elevated, you may have

purge excess grease before making adjustments.

If you have determined the appropriate course of actionis to re-tension the drivgo ahead and tighten the belts. Just be sure to use the Deflection Method so the belts are

tightened to established standards, or document your work so that YOU can establish those

standards. Always remember: the correct belt tension is just enough

tension to keep the belt from slipping under normalload

conditions.

145511306 belt tensioning

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